Method of electrowinning metals

ABSTRACT

Novel electrode for oxygen evolution comprising an electroconductive base provided with an outer coating containing a mixed crystal material of tantalum oxide and iridium oxide and preparation and use thereof.

United States Patent De Nora et al.

[ Dec. 16, 1975 METHOD OF ELECTROWINNING METALS Inventors: Oronzio De Nora; Giuseppe Bianchi; Antonio Nidola; Giovanni Trisoglio, all of Milan, Italy Assignee: Electronor Corporation, Panama City, Panama Filed: Jan. 31, 1975 Appl. No.: 546,112

Related US. Application Data Division of Ser. No. 36l,022, May l7, 1973, Pat. No. 3,878,083.

Foreign Application Priority Data May 18, 1972 Italy 24526/72 US. Cl 204/105 R; 204/290 F [51] Int. Cl. C25C 1/00; C25B 11/08 [58] Field of Search 204/105 R, 290 F References Cited UNITED STATES PATENTS 4/1975 De Nora et al 204/290 F Primary E.\'aminerR. L. Andrews Attorney, Agent, or FirmHammond & Littell [5 7] ABSTRACT 6 Claims, No Drawings METHOD OF ELECTROWINNING METALS This is a division of Ser. No. 361,022, filed May 17, 1973, now US, Pat. No. 3,878,083

STATE OF THE ART In various electrochemical process such as, for example, in the production of chlorine and other halogens. the production of chlorates, the electrolysis of other salts which undergo decomposition under electrolysis conditions and other electrolysis processes, it has recently become commercially possible to use dimensionally stable electrodes in place of graphite. These dimensionally stable electrodes usually have a film forming valve metal base such as titanium, tantalum. zirconium, aluminum, niobium and tungsten, which has the capacity to conduct current in the cathodic direction and to resist the passage of current in the anodic direction and are sufficiently resistant to the electrolyte and conditions used within an electrolytic cell, for ex ample, in the production of chlorine and caustic soda, to be used as electrodes in electrolytic processes. In the anodic direction, however, the resistance of the valve metals to the passage of current goes up rapidly, due to the formation of an oxide layer thereon, so that it is no longer possible to conduct current to the electrolyte in any substantial amount without substantial increase in voltage which makes continued use of uncoated valve metal electrodes in an electrolytic process uneconomical. I

It is, therefore, customary to apply electrically conductive electrocatalytic coatings to these dimensionally stable valve metal electrode bases. The electrode coatings must have the capacity to continue toconduct current to the electrolyte over long periods of time without becoming passivated and in chlorine production. must have the capacity to catalyze the formation of chlorine molecules from the chloride ions at an anode. They must be electroconductive and electrocatalytic and must adhere firmly to the valve metal base over long periods of time under cell operating conditions.

The commercially available coatings contain a catalytic metal or oxide from the platinum group metals, i.e., platinum, palladium, iridium, ruthenium, rhodium, osmium and a binding or protective agent such as titanium dioxide, tantalum pentoxide and other valve metal oxides in sufficient amount to protect the platinum group metal or oxide from being removed from the electrode in the electrolysis process and to bind the platinum group metal or oxide to the electrode base. The binding and protective metal oxide is usually in excess of the platinum group metal or oxide. Anodes of this nature have been described in British Pat. No. 1,231,280.

In anodes for the recovering of metals from uses by electrowinning, a continual source of difficulty has been the selection of a suitable material for the anode. The requirements are insolublity, resistance to the mechanical and chemical effects of oxygen liberated on its surface, low oxygen overvoltage, and resistance to breakage in handling. Lead anodes containing 6 to 15 per cent antimony have been used in most plants. Such anodes are attacked by chloride if present in the electrolyte. This is the case at the huge plant at Chuquicamata, Chile, where it is necessary to remove cupric chloride dissolved from the ore by passing the solution over cement copper, reducing the cupric to insoluble cuprous chloride. At this plant there was also developed an anode ofa coppensilicon alloy. called the Chilex anode, used in a portion of the tank-room, It has a longer life but raises the power consumption because of greater resistance and greater oxygen overvoltage.

Attempts to use mixed oxide coatings such as RuO- TiO for oxygen evolution have not been satisfactory in commercial use because passivation takes place after 200 to 1000 hours of operation at a current density of 1.2 KA per m The use of a Ta. O RuO mixed oxide coating improves the electrocatalytic activity and the life of the anode somewhat but not enough for commercial use. The use of a TiO lr.o coating has lower electrocatalytic activity.

OBJECTS OF THE INVENTION It is an object of the invention to provide a novel anode for oxygen evolution having an outer coating containing a mixed crystal material containing tantalum oxide and iridium oxide.

It is an additional object of the invention to provide a novel electrode with an outer coating of tantalum oxide and iridium oxide doped to improve the catalytic activity for oxygen evolution.

It is another object of the invention to provide novel electrodes having a coating of a mixed crystal to Ta O -,IrO on a valve metal alloy base having improved mechanical stability.

It is a further object of the invention to provide a novel process for the electrowinning of metals.

These and other objects and advantages of the invention will become obvious from the following detail description.

THE INVENTION The novel electrodes of the invention are comprised of an electroconductive base provided with a coating over at least a portion of its outer surface of a mixed material of tantalum oxide and iridium oxide. The coating may be as little as 5% of the outer surface of the electrode but preferably covers 50 to of the active face of the electrode. The preferred ratio of tantalum to iridium calculated in percent of metal is 1:1 to

The electrode base may be made of any electroconductive material such as metals and alloys thereof such as iron, nickel, lead, copper, etc but is preferably a valve metal such as tungsten, titanium, tantalum. niobium, aluminum or zirconium or alloys of 2 or more of said metals. The valve metal bases may be provided with an intermediate layer such as an oxide of the valve metal or a coating of another metal such as platinum group metals. The base may be a valve metal and at least either one metal having a low hydrogen overvoltage such as alloy of titanium with iron, cobalt, nickel, palladium, vandadium or molybdenum, or mixtures of two or more of said metals; or one metal suitable to form with titanium a protective oxide film even in acid solution such as an alloy of titanium with niobium, tantalum, zirconium or mixtures of two or more of said metals.

In a preferred embodiment of the invention, the electroconductive base is an alloy of a valve metal with a platinum group metal which as an improved corrosion resistance to acid electrolytes encountered in the use of the electrodes such as 5 to 15% sulfuric acid or 1 to 5% hydrochloric acid. A particularly useful alloy is titanium containing 0.1 to 0.20% by weight of palladium.

3 This corrosion resistance of the support of the coating prevents chipping off of the coating even if the anode is immersed for a few hours in an acidelectrolyte without anodic polarization.

In a'modification of the invention. the coating containing tantalum oxide and iridium oxide can be doped with an oxide of a metal with a valence of less than +4 to increase the catalytic activity for oxygen evolution without adversely effecting the mechanical properties of the coatings.

Without wishing to be limited to the following theoretical discussion. it is'believed that the conductivity of the Ta O -.IrO- system is of the n type and that the addition of the doping metal oxide reverses the type of conductivity from :1 type to 1 type" which improves the anodic process by producing electronic holes.

The doping metal oxide may be present in the coating in amounts ranging from 0.5 to 5.0% preferably 1.5 to 3.0% by weight of the said system calculated as metal. Examples of suitable doping metal oxides are alkaline earth metals such as calcium. magnesium. barium and mebers of Groups'VIII. VI B and VII B of the periodic Table such as cobalt. iron and nickel. chromium. mo-

verse effect onthe mechanical properties of the coatings as there is no coating loss in either instance even after 8000 hours operation.

The electrodes of the invention are particularly useful for electrolytic processes such as cathodic protection, electroflotation. organic electrosynthesis such as hydrodimerization of acrylonitrile and most particularly the electrowinning of metals. The said electrodes have a high electrocatalytic activity and a very los passivation rate of a few millivolts per month at a current density of 1.2 to 2.0 KA per m and a negligible weight loss if kept under anodic polarization.

The novel method of the invention for the preparation of the electrodes of the invention comprises applyingto an electroconductive electrode base a solution of a thermally decomposible compound of tantalum and a thermally decomposible compound of iridium. drying the coated electrode base by evaporation of the solvent and then heating the dried electrode base in the presence of an oxygen containing gas such as to form the desired electrode.

The heating step is preferably effected at temperatures of 350 to 600C. the optinum temperature being 500 to 550C. At temperatures below 350C, the oxidation is not completed or. requires too long heating time and at temperatures about 600C. the electrode base is likely to. be subjected to distortions and/or destruction by the high temperatures. 1

The preliminarydrying step is preferably effected by gentle heating in air to evaporate the solvent code- 7 posit the metal compounds. However. any convenient procedure may be used to remove the solvent such as standingunder reduced pressure.

In a preferred embodiment of the process. the coating is applied inmultiple coats with short periods of intermediate heating such as 500 to 550C for to minutes with a longer final heating after the last coat such as 500 to 550C for 45 minutes to l /2 hours. The

4 coating obtained thereby is very adherent and quite uniform. p I

Thelelectrode's of this'invention'arenparticularly useful for electrowinning process used in the production of various metals because they do not add impurities to the bath which deposit on the cathode, with the metals being won. and thereby contaminate the refined metal. as do anodes of for example load containing antimony and bismuth which give impure cathode refined metals. Moreover. their resistance to the'acid solutions and oxygen evolution and theirexc ellent anode potential makes them desirablefor this use.

n the following examples there are described several preferred embodiments to illustrate the invention. However. it should be understood that the invention is not intended to belimited to the specific embodiments.

EXAMPLE I 24 Titanium plates 10 mm' by 10 mm were etched in boiling 20% hydrochloric acid for sixty minutes and were then throughly w ashed with water. The plates were then coated with an aqueous solution of the compositions of Table I in 12 to 15 coats. After the applicationof each coat. the plates were d'ried and then heated for 10 minutes at 450C to 600C in an'oven with forced air circulation and then allowed to air cool. After the last coat. the plates were heated in the oven at the same temperature for 1 hour and were then air cooled. The values of Table I are calculated'as weight of free metal. The tantalum chloride was used as a solution in 20% hydrochloric acid. i

TABLE I.

Coating composition heating temp Sample No. in mg in C A, Tact. m 450 A lrCL, m 500 A 550 A4 600 B, TaCI 4 13.10 450 B. lrCL, 16.0 500 B. 550 B. 600 C1 TuCl- 10.70 450 C;- IrCL, 16.0 500 1': 550 C I (101) D, TaCI,-, 0 as 450 D. IrCL, v 16.0 500 D -550 D 600 E TaC1 6.85 450 E: IrCl 16.0 500 E;, I I 550 E. l V 600 F TUCI; 5.46 450 F IrCl.,- 16.0 500 F... .550 F. "600 TABLE I 0 Sample Anode Potential V (NHE) Coating weight No. initial after after loss value 3000 hs. 6000 hs mglcm" A 1.50 1.68 1.77 0.2 'A: 1.52 1.62 1.70 0.0 .A I 1.52 1.62 0.0

TABLE II-continued TABLE IV Sample I Anode Potential V (NHE) Coating weight Sample Anode potential (NHE) in Volts after coating loss No. inltial after alter loss No. initial 600 h 1000 h 1200 h in mg/cm" value 3000 has. 61100 hs mg/cm I r 5 1 1.411 2.00 35 A. 1.5: 1.62. 1.77 0.0 2 1.47 1.95 12 0' g B. 1.51 1.63 1.73 0.6 3 1.46 1.05 3.00 0 B 1.51 1.62 1.614;. 0.0 1.45 170 2.00 0 a. 1.53 1.62 1.68 0.0 5 1.52 1.03 1.146 0 B. 1.51 1.63 1.70 0.0 6' 1.5: 1w; 1.03 0 C. 1.50 1.65 1.70 0.3 7 1.51 1.111 ,1.1 5 0 C. 1.51 1.63 1.64 0.0 10 s v 1.52 1.115 1.00 0 C. 1.52 1.511 1.63 0.0 C. 1.52 1.63 1.73 0.0 D: 149 1.61 1.614 0.5 g The results of Table IV show that RuO TiO- 3 y 1 D{ 5 coated electrodes become passivated after only 1000 E. 15 hours and the Ta- O -,-RuO coated electrodes are only 1 I m; slightly improved and the TiO IrO coated electrodes E. 1.52 1.63 1.67 0.5 are no better. F 1.40 1.56 1.73- 0.0 F. 1.47 1.53 1.74 0.6 1 1.47 1.5: 1.77 1.2 1 EXAMPLE 114; H111 I3 10 plates of titanium plates containing 0.15% of pal- The results of Table 11 show that the electrodes of the invention have high electrocatalytic activity and a very low passivation rate and that the weight loss of the coating is negligible when within the limits of the invention. It should be noted that the ratio of Ta to Ir for samples F to F is'about' 0i34 Optimum values are obtained in the heating range ,of 50055OC.

EXAMPLE Il- For comparative purposes, electrodes were prepared as follows. Titaniumplates 10 mm by. 10 mm were etched in boiling 20%. hydrochloric acid for s ixty minutes and were then thoroughly washed with water. The plates were then coated with an aqueous solution of the compositions of Table III in 12 to 15 coats. After the application of each coat, the plates were dried and then heated for 10 minutes 'at 450C to 550C in an oven with forced air circulation and then allowed to air cool.

After the lastcoat, the plates were heated in the oven at the same temperature for l hour'and were then air cooled. The values of Table III are calculated as weight of free metal.

The anode potential for each anode was then determined by electrolysis of 10% by weight sulfuric acid at 60C at a current density of 1.2 KA/m The: initial anode potential and the anode potential after 600. 1000 or 1200 hours are reported in Table IV. The final loss of the coating was determined atthe end of the test.

ladium 10 X 10 mm) were sandblasted and then etched in refluxing 20% hydrochloric acid for 60 minutes. The plates were then coated with the compositions of Table V. The compositions were applied'in 15 to 20 coats with intermediate heating at 450C for 10 minutes in an oven with forced air circulation and cooling in air. The final heatingwas effected at the'temperatures in Table V for l hourfollowed by air cooling.

TABLEHV.

Coating compositions iri mg of Final healing Sample No. free metal in C AA TuCl 16+ 5110 BB IrCK, .16 5511 CC TaC1,-, -13.10+ 500 DD lrCL. 1 .0 550 12 TaCL, 1070+ 500- FF 0C1. 16.0 550 GG TUCI; XhU-t- 5111) HH 0C1. 16 1 1 550 :11 TaCl,-, 6115+ 1 500 11 0C1. 16 550 The anode potentials and coating'weight losswere determined as in Example I and the results are reported in Table VI.

i TABLE v1 The results of Table VI show that the electrodes of the invention with a titanium palladium alloy base have excellent electrocatalytic activity and low passiv ation rates.

EXAMPLE IV To demonstrate the improved corrosion resistance of a titanium palladium alloy. 10 plates made of titanium containing 0.15% by weight of palladium (10 X 10 mm) were sand-blasted and then etched in refluxing TABLE VII Anode Potential V(NHE) Coating weight Sample initial loss in No. value 1000 hs. 2000 hs. mg/cm BB 1.51 1.66 1.67 (1.3 CC 1.5] 1.57 1.62 0.3 DD 1.52 1.58 1.60 0.3 EE 1.50 1156 1.58 0.3 FF 1.52 1.56 1.58 0.3 G6 1.51] 1.55 1.56 0.3 HH 1.52 4 1.54 1.56 0.3 11 1.52 1.54 1.55 0.3 .1] 1.51 1.55 1.55

The results of Table V11 show that the electrodes of the invention having a titanium palladium alloy base have excellent electrocatalytic activity and low passivation rates and the coating does not chip off even without anodic polarization.

EXAMPLE v 10 titanium plates (20 X 20 mm) were etched in refluxing 20% hydrochloric acid for 60 minutes and after being throughly washed with water. the plates were coated with an aqueous solution containing 2.01 mg (as free metal) of TaCl 3.2 mg (as free metal) of 1rCl and 0.0394 ml of hydrochloric acid. The solution was applied in 12 coats with intermediate heating and cooling and a final heating as described in Example 1.

The, coated titanium plates were used. as anodes in cells for. the recovery of zinc from an aqueous electrolyte containing 100 g/liter of Zn S0 (as free metal). sulfuric acid and 10 to 50 ppm of glue. The cathode was a pure aluminum'sheet with a smooth surface and the electrolyte gap was 10 mm. The current density was 500 A/m and the electrolyte temperature was 35C. The anode potential, loss of coating, zinc thickness on the cathode and the morphology of the zinc deposit are reported in Table V111.

The cathodic current efficiency was found to be 92 95% in all cases and the purity of the zinc deposit was 99.999970. 1

EXAMPLE V1 Using the procedure of Example V. 5 titanium plates (20 X 20 cm) were coated with the composition of Example V. The coated plates were used as anodes in a cell for recovery of copper from an aqueous electrolyte containing g/liter (as free metal) of CuSO and 10 g/liter of sulfuric acid and the cathode was a smooth steel plate. The electrolyte gap was 15 mm and the bath temperature was 60C. The current density was 500 A/m The anode potential. loss of coating and copper thickness and morphology of the copper deposit are reported in Table 1X.

TABLE 1X Test Anode potential Coating Cu deposit Cu deposit No. V (NHE) weight thickness morphology loss in mm 1 1.47 0 4.5 smooth The cathodic current efficiency was found to be 100% in all cases and the purity of the copper was 99.9999% EXAMPLE V11 16 titanium coupons (20 X 20 mm) were etched in boiling azeotropic 20% hydrochloric acid for 40 minvutes and were throughly washed. The coupons were then coated with the composition of Table X in 20 coats. After the first l9 coats. the coupons were heated in a forced air circulation oven at 500C and then were air cooled. The last heating was at 500 or 550C for 1 hour followed by air cooling.

0.0362 mls.

The samples were then tested in 10% sulfuric acid at 60C with an anodic current density of 1.2 KA/m to determine the anode potential and coating loss after 2500 hours. The results are shown in Table X1.

TABLE X1 Specimen Temperature Ca content Anode Potential Weight No. final heat b.w.t. initial after loss treatment value 2500 hs. mg/cm V(NHE1 1 500C 1 1.51 1.55 1A 1.51 1.56 1B 550C 1 1.51 1.56 1C 1.51 1.56 2 500C 2.5 1.50 1.51 2A 1.51 1.51 2B 550C 25 1.50 1.52

TABLE Xl continued Specimen Temperature Ca content Anode Potential Weight No. final heat '7 h.\\'.t. initial after loss treatment value 2500 hs. mg/cm" V(NHE) 313 550C 4.0 1.51 1.55 3C 1.51 1.58 4 500C 5.0 1.51 1.60

4A 1.53 1.60 4B 550C 5.0 1.52 1.65 4C 1.52 1.65

EXAMPLE VIII Using the procedure of Example VII. X 20mm titanium coupons were coated with the composition of Table XII with the same heatings.

TABLE XII Specimen Liquid coating per amount No. each titanium sheet coupon gr/m l.lA lB.lC TaCl 2.077 mg. as Ta lrCl 3.200 mg. as [r 16 The anode potentials and the coating losses after 8000 hours was determined as in Example VII and the results are reported in Table XIII.

TABLE XIII Speci- Temper- Co content Anode Potential Coating men ature '72 h.\\'.t. initial after weight final value 8000 hs. loss heat VlNHEl mg/cm" treat ment 1 500C 1 1.52 1.5h 0 1A 1.52 1. 6 1B 550C 1.52 1. 7 1C 1.52 1.57 2 500C 25 1.52 1.53 2A 1.52 1.53 2B 550C 1.52 1.54 2C 1.52 1.54 3 500C 4 1.52 1.56 3A 1.52 1.56 3B 550C 1.52 1.56 3C 1.52 1.56 4 500C 5 1.52 1.56 4A 1.52 1.57 4B 550C 1.52 1.56 4C 1.52 1.57

Various modifications of the electrodes and processes of the invention may be made without departing from the spirit or scope thereof and it should be understood that the invention is to be limited only as defined in the appended claims.

We claim:

1. A method of electrowinning metals comprising passing an electric current through an anode having an electro'conductive base provided with a coating over at least a portion of its outer surface of a mixed material of tantalum oxide and iridium oxide. then an acid aqueous electrolyte containing the metal to be electrowinned and then the cathode on which the electrowinned metal is deposited. the ratio of tantalum or iridium calculated as metal is 7:1 to 0.34 to 1.

2. The method of claim 1 wherein the electro-conductive base is a valve metal.

3. The method of claim 2 wherein the valve metal is titanium.

4. The method of claim 1 wherein the electro-conductive base is titanium alloyed with up to 0.2% by weight of palladium.

5. The method of claim 1 wherein the coating also contains an oxide of a doping metal selected from the group consisting of alkaline earth metal. cobalt. iron, nickel, chromium, molybdenum and manganese.

6. The method of claim 5 wherein the doping metal is selected from the group consisting of cobalt and cal UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,926 Dated December 16, 1975 Oronzio De Nora et a1. Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 23, "mebers" should read members line 39, "105" should read low -I. Column 10, line 32,

"7" should read l Signed and ,Scalcd this [SEAL] Arrest.

RUTH C. MASON Commissioner ofParenls and Trademarks Thirteenth Day of July 1976 

1. A METHOD OF ELECTROWINNING METALS COMPRISING PASSING AN ELECTRIC CURRENT THROUGH AN ANODE HAVING AN ELECTRO-CONDUCTIVE BASE PROVIDED WITH A COATING OVER AT LEAST A PORTION OF ITS OUTER SURFACE OF A MIXED MATERIAL OF TANTALUM OXIDE AND IRIDIUM OXIDE, THEN AN ACID AQUEOUS ELECTROLYTE CONTAINING THE METAL TO BE ELECTROWINNED AND THEN THE CATHODE ON WHICH THE ELECTROWINNED METAL IS DISPOSED, THE RATIO OF TANTALUM OR IRIDIUM CALCULATED AS METAL IS 7:1 TO 0:34 TO
 1. 2. The method of claim 1 wherein the electro-conductive base is a valve metal.
 3. The method of claim 2 wherein the valve metal is titanium.
 4. The method of claim 1 wherein the electro-conductive base is titanium alloyed with up to 0.2% by weight of palladium.
 5. The method of claim 1 wherein the coating also contains an oxide of a doping metal selected from the group consisting of alkaline earth metal, cobalt, iron, nickel, chromium, molybdenum and manganese.
 6. The method of claim 5 wherein the doping metal is selected from the group consisting of cobalt and calcium. 